Stainless steels and nickel alloys

Inside the fermentation and distillation areas of the Kingfisher Bioenergy plant, almost everything that touches the process is stainless steel: the fermenters, the yeast propagation tanks, the beer well, the beer column and rectifier trays, the heat exchangers and the evaporator. Stainless steel is chosen for two reasons. It resists the warm, mildly acidic, wet streams of an ethanol plant far better than carbon steel, and it does not shed rust into the process, which matters for fermentation, for the color and purity of the ethanol and for the distillers grains sold as animal feed. This lesson explains what makes stainless steel stainless, how the families of stainless steel differ, why some grades fail where others survive, and when the plant must move up to nickel alloys.

What makes steel stainless

Stainless steels are iron alloys with at least about 10.5 percent chromium. Chromium reacts with oxygen to form an extremely thin, tightly adherent film of chromium-rich oxide, only a few nanometers thick, on the surface. This passive film separates the metal from the environment and slows corrosion to a negligible rate. Unlike rust on carbon steel, the passive film repairs itself almost instantly if it is scratched, as long as there is enough oxygen or another oxidizer in the environment. The whole behavior of stainless steel follows from this film: where it stays intact, the steel is nearly immune; where something breaks it down locally and prevents it from healing, attack can be fast and concentrated.

Chloride ions are the passive film's main enemy. They adsorb on the film at weak points, such as inclusions and scratches, and can break it down locally, starting a pit. Inside the pit, metal dissolves, the solution becomes acidic and chloride-rich, and the pit grows while the surface around it stays passive. The same process starts more easily inside crevices, under deposits and under gaskets, where the solution is stagnant and oxygen cannot reach the metal to heal the film. Heat makes all of this worse. Lesson 8 explains pitting and crevice corrosion in detail; for now, remember that the resistance of a stainless steel to chloride depends on its chromium, molybdenum and nitrogen contents, and that temperature, chloride level, acidity and crevices decide whether a given grade survives.

The families of stainless steel

Stainless steels are grouped by their crystal structure, which depends on the balance of alloying elements. MEC-250 explains the crystal structures; here is what each family means in practice.

Austenitic grades dominate process plants. Type 304 (18 percent chromium, 8 percent nickel) is the general-purpose grade. Type 316 adds about 2 percent molybdenum, which greatly improves resistance to pitting and crevice corrosion in chloride. The L suffix in 304L and 316L marks a low-carbon version (0.03 percent maximum), now standard for welded equipment because it avoids sensitization, explained below. Higher-alloy austenitics, such as 317L, 904L and the 6 percent molybdenum superaustenitic grades, extend chloride resistance further for brackish water, bleach plants and hot salty process streams. Duplex grades combine high strength with good chloride resistance, and their strength lets designers use thinner walls, which partly offsets their higher price per kilogram.

Measuring chloride resistance: PREN

A useful screening number for chloride pitting resistance is the pitting resistance equivalent number (PREN), calculated from the composition in weight percent. A common form is

Molybdenum counts three times as much as chromium and nitrogen sixteen times, which is why small additions of these elements make such a difference. Typical values are about 19 for 304L, 24 to 26 for 316L, 30 for 317L, 35 for 2205 duplex and 904L, and above 40 for super duplex and the 6Mo superaustenitics. PREN is only a ranking tool. Two alloys with the same PREN can behave differently, and actual performance depends on microstructure, surface finish, welding and the service conditions. Laboratory tests such as the critical pitting temperature (CPT) test, which finds the lowest temperature at which an alloy pits in a standard chloride solution, give a more direct comparison.

For the thin stillage evaporator, Rafael and Theo look at the conditions. The stillage runs at up to about 90 °C in the first effect, with a pH near 4, chloride levels that vary from about 300 to 1,000 mg/L depending on the corn and the recycled water, and heavy deposits on the tubes between cleanings. The 304L tubes, with a PREN near 19, pitted under deposits. Rafael sets a screening target of PREN 34 for the replacement tubes, based on published critical pitting temperature data and on experience at other ethanol plants. The program below calculates PREN for the candidates.

# Pitting resistance equivalent number (PREN) for common grades
# PREN = %Cr + 3.3 (%Mo + 0.5 %W) + 16 %N   (typical mid-range compositions)
grades = {
    "304L":        {"Cr": 18.2, "Mo": 0.0, "N": 0.05, "W": 0.0},
    "316L":        {"Cr": 16.8, "Mo": 2.1, "N": 0.05, "W": 0.0},
    "317L":        {"Cr": 18.5, "Mo": 3.2, "N": 0.06, "W": 0.0},
    "2205 duplex": {"Cr": 22.4, "Mo": 3.1, "N": 0.17, "W": 0.0},
    "904L":        {"Cr": 20.0, "Mo": 4.4, "N": 0.05, "W": 0.0},
    "2507 duplex": {"Cr": 25.0, "Mo": 3.8, "N": 0.28, "W": 0.0},
    "6Mo super":   {"Cr": 20.0, "Mo": 6.1, "N": 0.20, "W": 0.0},
    "alloy C-276": {"Cr": 15.5, "Mo": 16.0, "N": 0.0, "W": 3.8},
}
def pren(c):
    return c["Cr"] + 3.3 * (c["Mo"] + 0.5 * c["W"]) + 16 * c["N"]

need = 34   # PREN screening target chosen for the hot evaporator duty
for g, c in sorted(grades.items(), key=lambda kv: pren(kv[1])):
    p = pren(c)
    flag = "meets" if p >= need else "below"
    print(f"{g:12s} PREN = {p:5.1f}  {flag} target {need}")

304L         PREN =  19.0  below target 34
316L         PREN =  24.5  below target 34
317L         PREN =  30.0  below target 34
904L         PREN =  35.3  meets target 34
2205 duplex  PREN =  35.3  meets target 34
2507 duplex  PREN =  42.0  meets target 34
6Mo super    PREN =  43.3  meets target 34
alloy C-276  PREN =  74.6  meets target 34

Only 904L, 2205 duplex and the higher alloys meet the target. That seems to contradict the life-cycle analysis in Lesson 1, which favored 316L for a tank. The difference is temperature and deposits: the tank in Lesson 1 holds stillage at about 70 °C with smooth walls and regular cleaning, while the evaporator tubes are hotter, carry deposits and have hot heat-transfer surfaces where chloride concentrates as water boils off. Materials selection is always service specific. For the evaporator, Rafael's recommendation will be 2205 duplex tubes, with a chloride integrity operating window and more frequent cleaning.

Sensitization and intergranular corrosion

When austenitic stainless steel is held in the temperature range of about 450 to 850 °C, as happens in the heat-affected zone next to a weld, carbon in the steel can combine with chromium to form chromium carbides along the grain boundaries. The metal next to the carbides is depleted in chromium, sometimes below the level needed for a passive film. The steel is then sensitized, and in many acids it corrodes rapidly along the grain boundaries while the grains themselves remain intact. Grains can fall out of the surface, and the attack can penetrate a weld zone quickly. This was a common failure in the early decades of stainless steel use.

Three remedies are used. Low-carbon grades (304L, 316L) contain too little carbon to form harmful amounts of carbide during normal welding. Stabilized grades (321 with titanium, 347 with niobium) tie up the carbon in stable carbides so it cannot take chromium. And solution annealing, heating the whole item to about 1,050 °C and quenching, dissolves carbides that have formed. Lesson 8 covers intergranular corrosion again as a form of localized attack.

Stress corrosion cracking of austenitic stainless steel

Austenitic stainless steels such as 304L and 316L can crack when three conditions occur together: tensile stress (residual stress from welding and forming is enough), chloride, and a temperature above about 60 °C. The cracks are fine, branched and often transgranular, and they can go through a wall with very little visible corrosion. Classic locations are hot process lines with chloride-containing condensate, the outside of hot stainless lines under wet insulation that has leached chloride, and evaporators like the plant's. Duplex and ferritic grades and nickel-rich alloys resist chloride stress corrosion cracking much better. Lesson 9 covers it in detail.

Fabrication and surface condition

Stainless steel's corrosion resistance depends on a clean, fully passive surface. Fabrication can spoil it. Heat tint, the colored oxide left next to welds, has a chromium-depleted layer beneath it and pits easily; it should be removed by pickling or mechanical cleaning. Free iron from carbon steel tools, grinding wheels or work tables can embed in the surface and rust, starting pits. Weld spatter, scratches and rough grinding create crevices. Specifications for stainless equipment therefore call for dedicated tools, pickling or other cleaning of welds, and a final passivation treatment, usually with nitric or citric acid, to remove free iron and help the passive film form. Rafael's inspectors also check that welds in hygienic areas are smooth and fully penetrated, with no crevices for deposits or bacteria.

Nickel alloys

When stainless steels are not enough, plants turn to nickel-based alloys. Nickel itself resists caustic soda extremely well, which is why nickel 200 and 201 are used in caustic evaporators. Alloy 400 (nickel-copper) resists hydrofluoric acid, seawater and many neutral salts. Alloy 600 and 625 (nickel-chromium, with molybdenum and niobium in 625) combine high-temperature strength with good corrosion resistance. The nickel-chromium-molybdenum C-type alloys, such as C-276 and C-22, resist an exceptionally wide range of oxidizing and reducing acids and chloride solutions, including hot, acidic chloride environments that pit every stainless steel. Alloy B-2 and B-3 (nickel-molybdenum) handle hydrochloric acid, but they fail rapidly if oxidizers such as ferric ions or dissolved oxygen are present.

Nickel alloys cost several times more than stainless steel, so they are usually reserved for the parts of a system where the environment is worst: the dilution point of a sulfuric acid system, a heat exchanger in hot acid chloride service, or the weld overlay on a carbon steel vessel. Cladding, a thin layer of alloy bonded to carbon steel plate, gives the corrosion resistance of the alloy with the strength and economy of carbon steel. Clad plate and weld overlay are common in large vessels.